Sound recording apparatus



y 1940- G. L. DIMMICK 2,206,393

550mm RECORDING APPARATUS Filed March 31, 1959 2 Sheets-Sheet 1 Eli-.262

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Ennentor y 1940- G. L. DIMMICK 2,205,393

SOUND RECORDING APPARATUS Filed March 31, 1939 2 Sheets-Sheet 2 Eq- 3a 151a- 55.

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G/IL. pzrzzcr/ou 0 100% 6195. DE 55 C 7'/ ON Enoentor Glenn L. DLmmLciiGttorneg Patented July 2, 1940 UNITED STATES PATENT OFFICE SOUNDRECORDING APPARATUS Glenn L. Dimmick, Haddonfield, N. J., assignor toRadio Corporation of America, a corporation of Delaware This inventionrelates to sound recording apparatus, and more particularly to apparatusfor the photographic recording of sound on film by the variable densitymethod. The subject matter of the present application is an improvementon the apparatus of my Patent No. 2,095,317, issued October 12, 1937,and on the joint application of J. L. Underhill and myself, Serial No.210,592, filed May 28, 1938. In the aforesaid patlc ent, an apparatus isdescribed and claimed for recording sound by means of a penumbra orshadow of variable intensity which is cast by a fixed mask. In the saidjoint application, a modification of the apparatus is described and J15claimed wherein a movable penumbra mask is provided which is moved inaccordance with the envelope of the sound wave currents in order toprovide ground noise reduction.

In the apparatus according to the present invention, I provide means forcorrecting the quantity of light impressed upon the film at difierentpositions of the galvanometer mirror so as to compensate for thecharacteristic curves of the negative or positive film, or both, or forcomg pensating for any other non-linear efiect which may beproduced,such, for example, as nonlinearity of the amplifier or the like. Iaccomplish this result by the insertion of a mask having an aperture ofvariable area adjacent the ,n objective lens or mirror or filament ofthe recording optical system. The area of the mask is so adjusted as toproduce the necessary correction or compensation in the quantity oflight transmitted to the film without afiecting the area of the imageupon the film.

One object of the invention is to provide an improved sound recordingoptical system.

Another object of the inventionis to provide an improved record opticalsystem which will a be readily adjusted to compensate for irregularitiesin photographic characteristic or other non-linear characteristics.

Another object of the invention is to provide an improved optical systemof the penumbra 45 type wherein the brightness of the image cast uponthe film may be adjusted as desired.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from a reading of the following specification and an50 inspection of the accompanying drawings in which:

Figure 1 is a schematic perspective view of an optical systemconstructed in accordance with the present invention;

55 Figure 2a is a view of the slit with the penumbra image in oneposition thereon and Figure 2b shows the filament image in thegalvanometer as it appears in the position 2a of the penumbra on theslit;

Figure 2c shows the penumbra image in a sec- 5 end position in relationto the slit and Figure 2d shows the filament image in the galvanometermirror as it appears in the second position of the penumbra;

Figure 2e shows the penumbra in a third position in relation to the slitand. Figure 21 shows the galvanometer mirror as it appears from the slitin the third position;

Figure 3a shows a graph of light intensity plotted against galvanometerdeflection using a 16 rectangular galvanometer mirror and a mask havinga rectangular aperture;

Figure 3b shows a graph of light intensity plotted against galvanometerdeflection using a triangular mirror or mask shape; 20

Figure 4 is a typical H and D curve showing log exposure plotted againstprint density;

Figure 5 is a graph showing transmission of the sound record printplotted against the exposure of the negative required to produce thattransmission and illustrating the type of error which must becompensated for;

Figure 6 is a graph showing negative exposure required to produce alinear print plotted against the galvanometer deflection;

Figure 7 shows print transmission plotted against galvanometerdeflection for a print produced from a negative recorded with theoptical system corrected as shown in Fig. 6; and

Figure 8 shows the shape of mask required to produce the correctionindicated in Fig. 6.

Referring first to Fig. 1, the usual recording lamp filament isindicated at Ill. Light from the lamp 18 passes through the condenser IIto the rectangular mask l2 and thence to the movable penumbra mask l3,which may be moved in accordance with the envelope of the sound waves asdescribed in the Underhill et al. application supra. An additionalpenumbra mask I4 is provided at the extreme limit of desired movement ofthe mask l3 so that the mask l3 may move beyond the optical limitingpoint without further affecting the image produced. Light passing themasks l3 and I4 is directed by the lens I5 to the galvanometer mirrorI'B which 5 is adapted to be vibrated in accordance with the sound wavesto be recorded. The focal lengths of the lenses H and [5 are so chosenthat the lamp filament ID is imaged in the mirrow [6.

The lens I! directs light from the galvanom- 5 eter mirror I6 throughthe slit I9 in the slit plate I8 and through the mask 20 having theaperture 2! onto the objective lens 22. The lens I? is of such focallength that a real image of the galvanometer mirror i8 is formed in theplane of the mask plate 20 which is substantially in the plane of theentrance pupil of the objective 22. The objective 22 may, of course, becomposed of a plurality of lenses with the mask '28 closely adjacent thefirst of the lenses. The objective 22 is of such focal length as to forma sharp image of the split I9 upon the film 23.

It will be apparent from the: above stated relation of the severalelements of. the optical system that, as considered from the image onthe film 23, the mask plate 2!! with its aperture M is located at anaperture stop of thesystem, the slit I9 which is quite small is locatedas an image stop, the galvanometer mirror I6 is located as a second.aperture stop, the penumbra mask i3 is located as a third aperture stopand the mask I2 is located as a second image stop.

1e aperture stops affect the brightness of the image on the film 23without aifecting its size or position; whereas, the image stops serveto define the size and position of the image, the slit l9 defining theposition of the final image on the film 23 and the stopv l2 defining theintermediate image upon the slit plate I8.

The relation of these stops may be better understood by reference toFigs. 2a to 2 When the penumbra is in the position shown in Fig. 2a andthe image of the aperture stop in the plate I2 is in the positionindicated at 24 in Fig. 2a the slit I9 receives the full brightness fromthe filament ID and if the galvanometer mirror It is viewed through theslit 59, it will appear as shown in Fig. 227, the galvanometer mirrorbeing substantially filled with the filament image and the entire heightof the filament image being visible in the galvanometer mirror.

If the penumbra. image is moved to the position shown in Fig. 20 wherethe image of the aperture stop 12 occupies the position shown at 24 inthis figure inrelation to the slit !9, the light intensity on the slitwill only be half that which occurs in Fig. 2a. If, in this position,the galvanometer is viewed through the slit, it will appear as shown inFig. 211, only halfof the filament image being visible in thegalvanometer mirror, and this filling only half of the mirror area.

If the galvanometer mirror I is further tilted until the penumbra iscompletely ofl the slit I3, as shown in Fig. 2e, then observationthrough. the slit will show the galvanometer mirror as indicated in Fig.2 with no light being directed from the galvanometer mirror to the slitand with none of the filament image being apparent. This ef-- feet ispartly due to the narrow Width of the slit l9 since an aperture at thispoint of a considerable area would permit light to pass from the otherportions of the shadow; or it might be stated that the slit serves toscan the penumbra image as it moves up and down.

If the aperture plate 20 were omitted or if a rectangular aperture wereprovided therein similar to the aperture in the plate I2, then the lightintensity as varied by deflection of the galvanometer mirror wouldfollow the curve shown in Fig. 3a, which shows that the light intensityvaries linearly with the galvanometer deflection from minimum to maximumintensity.

If a triangular mirror or triangular mask were substituted as shown inFig. So, then the light intensity reaching the film would vary as thesquare of the galvanometer deflection substantially as shown in thecurve of light intensity plotted against galvanometer deflection in Fig.3b.

If a typical film stock such as is used for making sound picture printsis exposed to light of different intensities and developed, and thedensity is plotted against the exposure as in the conventional H and Dcurve, a curve such as shown in Fig. 4t- Will result; 1. e., Fig. 4:represents a typical H and D curve for positive stock. It will beapparent from this curve that if the light to be transmitted by theprint onto the photccell in reproduction is to be a linear function ofgalvanometer deflection, then the light transmitted to the film for itsexposure must not be a linear function of galvanometer deflection asiown in Fig. 3a, but must vary inversely to the shape of the curve shownin Fig. l. The negative film stock on which the original recording ismade will likewise have a characteristic curve somewhat similar to, Fig.4, but since the points of greatest transmission on the negative arethose of least transmission on the print and vice versa, the negativecurve will produce a curvature of print density or transmission which isinverted in respect to the print curve.

The composite of the negative and print curves is shown in Fig. 5, wherethe transmission of the print is plotted against the exposure of thenegative. It will be apparent that in order to secure a substantiallylinear print transmission in relation to galvanometer deflection, thelight must be so varied. in relation to galvanometer deflection toproduce a curve which is substantially complementary to the curve shownin Fig. 5 and such a curve is shown in Fig. 6, where the galvanometerdeflection is plotted against the required negative exposure for asubstantially linear print transmission.

In Fig. '7 the galvanometer deflection is shown as plotted against printtransmission when corrected in the manner shown in Fig. 6, and it willbe apparent that a substantially linear relationship results.

In order to accomplish the foregoing substantially linear relationshipbetween galvanometer deflection and print transmission, I insert themask 20 having the aperture 2! as shown more accurately in detail inFig. 8. This aperture 2| is designed in accordance with the requirednegative exposure as indicated in Fig. 6.

Since the mask 2! is located at the entrance pupil of the objective 22and the galvanometer mirror I E is imaged thereon, the image on the mask20 will vary in the manner indicated in Figs. 2b, Zdand 2f. It will beapparent that at the upper end of the aperture, which is relativelynarrow, only a small amount of light will be transmitted from thefilament, while at the lower portion, which is broader, a greater amountof light will be transmitted and by properly shaping this aperture, thelighttransmitted from the galvanometer for any given deflection. willcorrespond substantially with the curves shown in Fig. 6.

It will be apparent that any movement of the mask I3 will produce ashiftin in the portion of the filament visible on the mask plate 28 in Pthe exposure be varied by the galvanometer It} or by the mask l3.

It will be apparent to those skilled in the art that a mask of variabledensity may be substituted for the aperture plate 20 and will accomplishthe same result, but I prefer to use the variable area mask because theshape of the aperture may be readily changed as required to accommodateemulsions of different characteristics, and the shape of the aperturemay be readily calculated or plotted from a curve of print transmissiondetermined in the manner above described.

It will also be apparent that the mask may be located at any of theother aperture stops of the system, as, for example, at the galvanometermirror or adjacent the filament of the light source.

Having now described my invention, I claim:

1. In a sound recording optical system of the penumbra type having anobjective, a mask having light transmission varying along the directionof movement of the light beam located at the entrance pupil of theobjective.

2. A sound recording optical system including an exciter lamp, means fordirecting a beam of light from said exciter lamp to a means fordeflecting the beam in accordance with sounds to be recorded, a penumbramask between said lamp and said deflecting means, means having a narrowslit therein between said deflecting means and the sound recordingposition, an objective for imaging said slit upon a film at the soundrecording position, and a mask having a transmission varying along thedirection of deflection of said beam located substantially at theentrance pupil of said objective.

3. A sound recording optical system including an exciter lamp, means fordirecting a beam of light from said exciter lamp to a means fordeflecting the beam in accordance with sounds to be recorded, a movablepenumbra mask between said lamp and said deflecting means, means havinga narrow slit therein between said deflecting means and the soundrecording position, an objective for imaging said slit upon a fllm atthe sound recording position, and a mask having a transmission varyingalong the direction of deflection of said beam located substantially atthe entrance pupil of said objective.

4. A sound recording optical system including an exciter lamp, means fordirecting a beam of light from said exciter lamp to a galvanometer fordeflecting the beam in accordance with sounds to be recorded, a penumbramask between said lamp and said deflecting means, means having a narrowslit therein between said deflecting means and the sound recordingposition, an objective for imaging said slit upon a film at the soundrecording position, and a mask having a transmission varying along thedirection of deflection of said beam located substantially at theentrance pupil of said objective.

5. A sound recording optical system including an exciter lamp, means fordirecting a beam of light from said exciter lamp to a galvanometer fordeflecting the beam in accordance with sounds to be recorded, a penumbramask between said lamp and said deflecting means, a slit plate having anarrow slit therein between said deflecting means and the soundrecording position, an objective for imaging said slit upon a film atthe sound recording position, and a mask having a transmission varyingalong the direction of deflection of said beam located substantially atthe entrance pupil of said objective.

6. A sound recording optical system including an exciter lamp, means fordirecting a beam of light from said exciter lamp to a means fordeflecting the beam in accordance with sounds to be recorded, a penumbramask between said lamp and said deflecting means, means having a narrowslit thereinbetween said deflecting means and the sound recordingposition, an objective for imaging said slit upon a film at the soundrecording position, and a mask having an aperture having a transmissionvarying along the direction of deflection of said beam locatedsubstantially at the entrance pupil of said objective.

'7. In combination with a sound recording optical system of the penumbratype having a light source, an objective for directing a narrow line oflight upon the film, a plurality of lenses for directing light from saidsource to said objective, and means for deflecting the light beam inaccordance with sound to be recorded, a mask having light transmissionvarying along the direction of deflection of the light beam.

8. In combination with a sound recording optical system of the penumbratype having a light source, an objective for directing a narrow line oflight upon the film, a plurality of lenses for directing light from saidsource to said objective, and means for deflecting the light beam inaccordance with sound to be recorded, a mask having light transmissionvarying along the direction of deflection of the light beam and locatedat one of the aperture stop positions of the system.

9. In combination with a sound recording optical system of the penumbratype having a light source, an objective for directing a narrow line oflight upon the film, a plurality of lenses for directing light from saidsource to said objective, and means for deflecting the light beam inaccordance with sound to be recorded, a mask having light transmissionvarying along the direction of deflection of the light beam and locatedadjacent said deflecting means.

GLENN L. DIMMICK.

